Dynamical magnetotropic susceptibility as a new probe of Kitaev materials and beyond
The magnetotropic susceptibility, $k(ω)$, probes ultra-low-frequency uniform ($\boldsymbol{q}=0$) spin and charge fluctuations in a crystal mounted on an oscillating cantilever: its real part shifts the oscillation frequency, while its imaginary part characterises the induced damping. We derive $k(ω)$ within linear response theory for a generic correlated-electron Hamiltonian, showing that its real part is sensitive to magnetic anisotropy and its imaginary part encodes the uniform dynamical spin susceptibility, even for spin-symmetric insulators, while in metals it reveals eddy-current damping conditions. Using auxiliary-field quantum Monte Carlo, we compute $k(ω)$ for microscopic models of $α$-RuCl$_3$, finding that the low-temperature $k(ω=0)/T$ scaling with $B/T$ is a signature of dominant Kitaev coupling, robust to optical phonons, while the dynamical response shows local-moment-like features. We highlight applications to Kondo destruction quantum criticality, relevant to strange metallicity and unconventional superconductivity.